Much research has been conducted in the area of cellulose hydrolysis to produce fermentable sugars, such as glucose therefrom. Cellulose is the most abundant polymer on earth, and is characterized as a straight chain polymer composed of glucose with beta 1,4-linkages. Cellulose may exist in crystalline or amorphous forms. Generally speaking, one can easily hydrolyze amorphous cellulose with dilute acid or enzymes. Crystalline cellulose, on the other hand, is difficult to hydrolize presumably due to a tight physical packing of the cellulose molecules. As a result, degradation of the hydrolysis products is significant as represented by the following scheme: ##STR1##
Various methods have been touted for decrystallizing cellulose through the use of solvents to precipitate it in an amorphous form. However, there methods all utilize cellulose which is solid, albeit amorphous.
Penque U.S. Pat. No. 4,018,620 describes a method of hydrolyzing cellulose using calcium chloride and dilute acid at a temperature of 100.degree. C. to form a colloid suspension of the cellulose which is the hydrolyzed at a temperature of 120.degree. C. for a period of 30 minutes. Contrary to Penque's findings, and due apparently to an error in the unit and chemistry of Penque's analysis, we have found that the claimed method does provide a complete conversion of cellulose to glucose. According to Penque, 10% (w/v) of newsprint (which contains cellulose and hemicellulose) was hydrolyzed, thereby obtaining a 10% (w/v) reducing sugar solution which is equivalent to 50% of the total reducing sugar.
Because the hemicellulose fraction is very easy to hydrolyze, and since newsprint generally contains at least 15% hemicellulose, one must subtract this value from the yield of glucose from cellulose fraction thereby getting a yield of only 20%. In addition, Penque used Clinitest tablets to quantitate the sugar. These tablets are also reactive to the degraded glucose, (Hydroxymethyl furfural) and do not provide a true reading of reducing sugars. On the other hand, analyzing with "Tes-tape" or glucose analyzer, which is specifically reactive to glucose, would provide a different and more accurate result.
It is thus, desirable to hydrolyze cellulose in a liquid state. Unfortunately, conventional cellulose swelling reagents and cellulose solvents are either too severe for glucose or unable to catalyze the cellulose hydrolysis.
Zinc chloride is known as a cellulose swelling reagent, and swells the cellulose at a concentration range from 60 to 80%, with maximum effect at 75% and 65%. The pH of ZnCl.sub.2 at this range on concentration is 0 to -2, and thus is able to provide a catalytic function of cellulose hydrolysis. However, under such conditions glucose is also degraded at a faster rate.
In our concurrently filed application Ser. No. 377,077, titled "Quantitative Hydrolysis of Cellulose to Glucose Using Zinc Chloride", we describe a method for hydrolyzing cellulose to pretreating same with concentrated zinc chloride to liquify the cellulose, thereafter reducing the zinc chloride concentration (e.g. by dilution) and completing acid hydrolysis to form glucose. While that process provides hydrolyzed yields of over 90%, the separation of zinc chloride and glucose is costly.
Accordingly, it is the primary object of the present invention to provide a means for effectively pretreating cellulose with zinc chloride and thereafter separating the zinc chloride from the glucose produced.
This and other objects of the present invention will be more apparent from the discussion which follows.